The present invention is generally directed to horizontal penetrations extending between the inner and outer walls of a cryostat, particularly a cryostat employing liquid helium as a coolant material. More particularly the present invention is directed to a plug tube employing heat transfer contacts to provide a refrigeration interface for a nuclear magnetic resonance (NMR) magnet cryostat.
In the generation of medical diagnostic images in NMR imaging, it is necessary to provide a temporally stable and spacially homogeneous magnetic field. The use of superconductive electrical materials maintained at cryogenic temperatures provides advantageous means to produce such a field. Accordingly, for such NMR imaging devices, a cryostat is employed. A cryostat contains an innermost chamber in which liquid helium, for example, is employed to cool the superconductive magnet materials. The cryostat itself, typically comprises a toroidal structure with other nested toroidal structures inside the exterior vessel to provide the desired vacuum conditions and thermal shielding. Since it is necessary to provide electrical energy to the main magnet coil, to various correction coils and to various gradient coils employed in NMR imaging, it is necessary that there be at least one penetration through the cryostat vessel walls.
Typical prior art penetrations have been vertical. While the apparatus of the present invention is particularly directed to non-vertical penetrations, it is also applicable to vertical ones. However, vertical penetrations have produced undesirable problems of alignment and assembly from a manufacturing viewpoint. Horizontal cryostat penetrations have, however, not been employed for reasons of thermal efficiency. In particular, it is seen that for a coolant such as liquid helium, there is a large dependency of gaseous or vapor density upon temperature. Accordingly, helium vapor found within a vertical penetration is naturally disposed in a layered configuration as a result of the density variation from the bottom to the top of the penetration. This layering provides a natural form of thermal insulation along the length of a vertical penetration. In particular, at any position along the axis of such a penetration, the temperature profile is substantially constant. However, this would not be the case for a conventional horizontal cryostat penetration since any layering that would exist would not be in the direction of the long axis of the cryostat penetration, that is, in the direction of the temperature gradient. Accordingly, the temperature gradient along the penetration would tend to set up free convection currents in the vapor within the penetration. This would result in a much more rapid loss of coolant than is desired. Since the cost of helium is relatively high, it is seen that this loss of coolant is undesirable.
However, cryostat plugs for horizontal penetrations have been designed so as to include special plug tubes with spiral grooves. These spiral grooves have been provided in a thin walled removable plug tube structure which is inserted into the cryostat penetration. Gasket material is disposed in the grooves so that a helical helium vapor path is provided along the axial length of the penetration with helium vapor flowing in the annular space defined between the plug tube and the cryostat penetration wall. This annular space must be closely controlled since the speed of helium vapor within the space determines in part the rate of heat loss from the cryostat. This annular space is typically about 1/100 of an inch wide. The spiral gas path for vaporized helium counteracts the natural convection currents that would otherwise be established in the annular volume.
However, along the axial length of the penetration it is desirable to be able to transfer a certain amount of thermal energy from certain cryostat vessel walls to the exterior environment. This is desired both for refrigerating various cryostat shields and also in those situations in which an exterior helium liquefier is provided to recycle helium vapor which is boiled off from the interior cryostat vessel or vessels. For example, it is desired to transfer between about 1 and 2 watts of thermal energy at an intermediate shield location which is maintained at a temperature of between about 10.degree. K. and about 50.degree. K. Additionally, it is desirable to be able to transfer between about 5 and about 10 watts of thermal energy at a more exterior shield which is maintained at a temperature of between about 50.degree. K. and about 100.degree. K. The annular gap, represents too large a thermal resistance for effective heat transfer. Additionally, the size of this gap and the relatively thin walled nature of the plug tube provides stringent limitations upon the size, strength and construction of any device that might be employed to improve local heat transfer conditions at the shield locations without adversely affecting either the size of the gap, strength of the tube or other thermal conditions. Additionally, the spiral gasketing material in the plug tube prevents any heat transfer augmentation device to be placed on a penetration tube wall where it would interfere with insertion of the plug. Also, during insertion and removal operations, air must be prevented from entering a penetration tube since there would be a tendency to form liquid or solid air condensate. Accordingly, a retractable disk or cylinder which slides in and out of a penetration tube is added to prevent entry of air after plug retraction. The retractable disk or cylinder generally requires a constant cross section tube in all practical construction designs.
Accordingly, the problem posed is briefly the following. A means is to be provided for localized thermal transfer across a high thermal resistance gap, typically filled with coolant vapors such as helium. The thermal transfer is to be localized at specific shield locations and cannot interfere with the insertion or removal of a plug tube partially defining the gap nor with the spiral flow of coolant vapor in the gap. Additionally, the desired thermal transfer structure must not interfere with the spiral gasketing material and must not interfere with the insertion and removal of the plug tube. Most critically, the thermal transfer means desired is to be employed in an extremely thin walled structure (15 to 25 mils) having significant length (approximately 14 inches) with respect to its diameter (approximately 3.5" in practice). Further reduction in the size of the annular gas gap, even locally, is not readily feasible because of the manufacturing tolerances involved. Additionally, significant increases in axial dimensions for the purpose of improved heat transfer area would significantly affect the overall dimensions of the entire cryostat structure and are generally considered to be economically prohibitive, particularly in light of the relatively inexpensive solution provided by the present invention.